More than 65 species of plants have gone extinct in the U.S. and Canada since European colonization, according to a new study.

The study was conducted by a group of 16 experts from across the U.S., including University of Washington Biology Professor Richard Olmstead. The study showed that far more plant species have gone extinct on the continent than previously documented.

Most of the plants that disappeared existed in the western parts of the country, and particularly the Southwest. Although Washington state did have two plants — the thistle milk-vetch and pale bugseed —- which went extinct in Eastern Washington.

Extinction likely occurred before the plants were analyzed by scientists, and were likely due to human impacts on changing land use, a press release from the University of Washington states. One reason suggested by the study for why more extinctions were found on the West Coast was that scientists had more of a change to examine plants before they were destroyed by human development.

Extinction rates have growing due to human causes. One study found plant and animal extinctions are happening.